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Chem
Aromatic compounds
| Question | Answer |
|---|---|
| Kekulé model | 6 Carbon ring joined by alternating single and double bonds |
| Delocalised model | Planar, cyclic, hexagonal hydrocarbon with sideways overlap of p-orbitals forming a delocalised pi-system with a ring of electron density above and below the carbons |
| Evidence to disprove Kekulé model | Unreactive - double bonds should allow for electrophilic addition and decolourise bromine water C-C bond length - in x-ray diffraction all bond lengths equal, C=C should be shorter Hydrogenation enthalpy - less exothermic than expected (-208>-360) |
| Nomenclature of aromatics | Benzene is parent chain with alkyl groups up to 6C, halogens, and nitro groups - these groups are prefixes to benzene Benzene is a substituent with alkyl chains with a functional group or 7C and greater - prefix is phenyl |
| Nomenclature exceptions | Benzoic acid Phenylamine Benzaldehyde |
| Nitration of benzene | Uses conc. HNO3 with conc. H2SO4 catalyst 1. HNO3 + H2SO4 -> (NO2)+ + (HSO4)- + H20 2. Electrophilic substitution 3. H+ + (HSO4)- -> H2SO4 |
| Halogenation of benzene | Uses a halogen in presence of a halogen carrier catalyst (FeX3 or AlX3) 1. Br2 + FeBr3 -> (FeBr4)- + Br+ 2. Electrophilic substitution 3. H+ + (FeBr4)- -> FeBr3 + HBr |
| Friedel-Crafts (alkylation and acylation) | Uses haloalkane or acyl chloride with halogen carrier catalyst 1. C2H5Cl + AlCl3 -> (AlCl4)- + (C2H5)+ 2. Electrophilic substitution 3. H+ + (AlCl4)- -> AlCl3 + HCl |
| Relative reactivity of benzene to alkenes | Alkenes have localised electrons in pi bond - area of high electron density - can induce a dipole making Br2 an electrophile Benzene has delocalised electrons in pi system - electrons density around 2 carbons less than in C=C - can't induce a dipole |
| Acidity of phenol compared with carboxylic acids | Phenols react with NaOH (strong base) but not carbonates (weak bases) Carboxylic acids react with NaOH and carbonates (strong and weak bases) |
| Bromination of phenols (electrophilic substitution) | Phenol + 3Br2 -> 2,4,6 - tribromophenol + 3HBr |
| Nitration of phenols (electrophilic substitution) | Phenol + HNO3 -> 2-nitrophenol or 4-nitrophenol + H2O |
| Reactivity of phenol compared to benzene | Lone pair on -OH group is donated into to delocalised pi-system which increases electron density. Now more susceptible to attack from electrophiles as it attracts them more strongly |
| Directing groups | -OH and -NH3 are activating groups as they are electron-donating so have a 2- and 4- directing effect NO2 is a deactivating group as it is electron-withdrawing so has a 3- directing effect |
| Importance of directing groups to organic synthesis | Can be used to ensure reactants are substituted to the correct position |
| Synthesis of TNT | Methylbenzene (toluene) with H2SO4/HNO3 at 50C to form 2-nitrotoluene At 70C further substitution creates 2,4-dinitrotoluene - higher temp as NO2 is deactivating Extreme conditions results in 2,4,6-trinitrotoluene |
Created by:
silver54331